Part of the problem with your heating is probably topping it up with water. Engine coolant / antifreeze has a higher boiling point than water. As you top it up and dilute it, your constantly lowering the boiling point of the fluid. When you have (as you may have done now) Watered it all the way down, it just takes about 10 minutes for the engine to get to 100 degrees operating temperature, and the water will vaporize and leak.

I would suggest for this you get new coolant and a good engine flush to clean it up, followed by a new expansion tank cap to keep it all air tight.

As for the paint, theres no reason you can, however, be prepared for a difference in shade, as the sun changes the colour, but if you take your time, apply even and repeated coats of paint (lightly) there is no reason you couldnt have those patches almost exactly as they were.